of Terrestrial Algae. II. 719 
features as the cells of drought material. The obviously dead cells in all 
cases looked quite different from the others. 
Only fresh material of Prasiola has been examined with this type of 
illumination. The cells showed pronounced milkiness of the protoplast, no 
Brownian movement, and usually no indication of vacuoles. In some of 
the longer cells, however, there appeared to be very small vacuoles around 
the periphery of the protoplast, which is contrary to what is usually 
described for this alga (cf. Fritsch, 1922 , p. 14), and may be due to the fact 
that the material examined had been completely submerged during the pre¬ 
vious days as the result of heavy rains. 
A comparison of fresh and drought material of protonema showed the 
same contrast as above described for the two filamentous algae: Brownian 
movement and marked vacuoles in the fresh cells, no movement and no 
vacuoles in the drought cells, which, however, appeared quite healthy. 
Material that had been submerged for two months in 5 per cent, sea-salt 
solution showed in the healthy cells near the tips of the branches (cf. p. 706) 
a very well-defined cytoplasmic lining which was typically milky through¬ 
out ; there was a clear vacuole in this case. 
In Pleiirococats , lastly, one recognizes no vacuoles, even in fresh material 
(cf. Fritsch, 1922 , p. 14); the protoplasts have a general milky appearance, 
but the large granules in the cells make it difficult to say whether there is 
any Brownian movement or not. It is probable, however, that there is none. 
The general conclusions to be drawn from these observations are that the 
granular cells of drought material still possess a healthy protoplast, which 
has, however, undergone a marked increase in viscosity, so that Brownian 
movement can no longer be recognized in it. It would appear, too, that an 
increase in the area occupied by the protoplast takes place at the expense 
of the vacuole. In all of these respects Pleurococcus and Prasiola , in the 
normal condition as found in nature, resemble the drought condition of the 
other forms, and to this many of their peculiarities and their great power of 
resistance may be due. Extreme viscosity of the protoplast may well 
account for the failure of the majority of the cells of drought material to 
undergo plasmolysis. On the other hand, the recovery from plasmolysis of 
strongly plasmolysed cells that have been immersed for long periods in 
hypertonic solutions, when mounted in tap-water (cf. p. 707), shows that the 
protoplasts of these cells, though otherwise usually showing similar charac¬ 
teristics to the protoplasts of cells in the drought condition, are not as 
viscous. The same remarks also apply to the fresh protoplasts of Pleuro - 
coccus and Prasiola , since plasmolysis of these takes place when high con¬ 
centrations are used. 
3 I! 
